1. Field of the Invention
This invention relates to a device and a method for radiation measurement, applied to monitor the radiation in an extensive range for improving resistance to noises in a digital signal processing.
2. Description of the Related Art
As far as radiation measurement is concerned, if wide range radiation is measured, then the pulse measurement method and the Campbell measurement method are often used together.
Generally, the pulse measurement method counts the pulse number of a pulse signal from a radiation sensor, but if the pulses overlap and it cannot count by the pulse measurement method, the Campbell measurement method is performed.
For example, from six to ten start-up range neutron monitor sensors (SRNM sensors) and from one hundred to two hundred local power range monitor sensors (LPRM sensors) are installed inside of a reactor pressure vessel containing nuclear reactor core to monitor nuclear reactor power. A start-up range neutron monitor and a power range neutron monitor measure outputs of the SRNM sensors and the LPRM sensors, respectively, to monitor the nuclear reactor power in a monitoring range of about eleven figures.
In this composition, the start-up range neutron monitor is used to count the pulse number of an output signal of the SRNM sensor in order to monitor relatively low reactor output, that is, the output is in from 10−9% to 10−4% of effective full power of the reactor. This is henceforth called the pulse measurement method.
On the other hand, the Campbell measurement method, that is, the measuring of fluctuation power generated due to overlapping of the pulse outputted from the sensor, is used in order to monitor relatively high reactor output, that is, the output is in from 10−5% to 10% of the effective full power of the reactor.
Hereafter, a conventional technical example of the pulse measurement method and the Campbell measurement method in a nuclear reactor start-up monitoring system, which is disclosed in Japanese Patent Disclosure (koukai) No. 2000-162366, which is equivalent to U.S. Pat. No. 6,181,761, is explained with reference to FIG. 18.
The nuclear reactor start-up monitoring system shown in FIG. 14 is composed of an SRNM sensor 1 for outputting an electric signal containing pulse components corresponding to the number of neutrons in response to neutrons generated in the nuclear reactor, an analog preamplifier 2, an A/D (analog-to-digital) converter 3, and a pulse counter 23, an integration counter 24, a power calculator 25, an arithmetic average calculator 26, and a reactor power monitoring system 27. The analog preamplifier 2 amplifies the electric signal having pulse components outputted from the SRNM sensor 1 to regularize the electric signal, and the A/D converter 3 converts an analog signal outputted from the preamplifier 2 to digital data sampled at intervals which are shorter than a pulse width of the pulse included in electric signal outputted from the SRNM sensor 1. The pulse counter (PC) 23 counts a number of pulses in the sampled data outputted from the A/D converter 3 and converts the number of the pulse to an output level value contained in relatively low range power of the nuclear reactor, and the integration counter 24 adds the sampled valve outputted from the A/D converter 3 to raise the measurement accuracy. The power calculator 25 calculates a power by squaring the added value of the integration counter 24, and the arithmetic average calculator 26 averages the power calculated by the power calculator 25. The reactor power monitoring system 27 continuously monitors the output at the start-up of the nuclear reactor based on the counter result of the pulse counter 23 and the calculation result of the arithmetic average calculator 26.
In the digital reactor start-up monitoring system of such a composition, the preamplifier 2 amplifies and regularizes a shape of a pulse included in the electric signal outputted from SRNM sensor 1, and the A/D converter 3 samples the amplified and regularized pulse at high speed and calculates the pulse by using one or more logical operations. Also, the pulse counter 23 counts the calculation results outputted from the A/D converter 3 as an output pulse of the sensor if each calculation result outputted from the A/D converter 3 is in a corresponding predetermined range, respectively.
On the other hand, the same sampled value is added in the integration counter 24 to lower into a level of a sampling rating required for the Campbell measurement method and to earn a dynamic range for improving the number of equivalent bits. The power calculator 25 adds square values of the results after performing band-pass-filter process for the results, and the arithmetic average calculator 26 averages the results calculated by the power calculator 25 and computes the Campbell output value. The pulse enumerated data and the Campbell output value are estimated by the nuclear reactor output evaluation unit 27 and are displayed as a nuclear reactor output.
In this composition, calculation limited to the sensor-outputting pulse can be carried out with excluding noises having long pulse widths by discrimination based on information of not only a pulse height of a pulse but a pulse width by the pulse calculator 23.
That is, in the reactor start-up monitoring system of FIG. 18, for example, the output signal of the SRNM sensor 1 containing a pulse with the pulse width of 100 nanoseconds is sampled at intervals of 25 nanoseconds.
Four sampled-data, from data No. k-3 to data No. k, denoted as S(k-3), S(k-2), S(k-1), and S(k) in order, respectively, which correspond to a pulse width, are used to calculation described below, as S(k-3) is a sampled value at a rise point of a pulse, S(k) is a sampled value at a fall point of the pulse, and two sampled data S(k-1), S(k-2) are in between S(k-3) and S(k). It considers a result Out(k) of this calculation as an index of pulse discrimination, and as a result, the pulse is counted as a neutron pulse if it is in a range of predetermined level.Out(k)={b*S(k-2)+c*S(k-1)}−{a*S(k-3)+d*S(k)}  (1),
where a, b, c and d are non-zero constants.
By this calculation, it becomes possible to calculate only signals having almost similar pulse widths as that of the output pulse of the SRNM sensor 1. That is, even if a large surge-like noise becomes overlapped on a signal pulse, it can count measured value exactly by deducting the ground level of the pulse.
In addition, by setting two or more indices such as the Out(k) for detecting a case corresponding to such a sensor pulse form as mentioned above and using AND logic among these indices, this discrimination performance can be improved further.
Thus, even if a surge-like noise with a pulse width of several microseconds overlaps, and is supposed to be guided into a pulse in an electric signal outputted from the SRNM sensor most easily, the surge-like noise can be removed nearly completely and a limited calculation of sensor pulses with a pulse width of about 100 nanoseconds can be performed.
On the other hand, in the Campbell measurement method, the power calculator 25 restricts a frequency band and calculates an average of square values of the sampled data. In this composition, since the frequency band can be set up by software programming, if a noise is in a certain frequency equivalent to a measurement band, changing the measurement band on the software programming can reduce guidance of the noise.
However, there are several subjects described below in the nuclear reactor start-up monitoring system according to the above-mentioned conventional technology.
A first subject concerns reduction of a bipolar noise. That is, if surge noise with a pulse width of several microseconds and sensor output pulse overlap, it is necessary to compute a value corresponding to a pulse peak value by using the difference between them in order to count the overlapped sensor output pulse without preparing dead time.
In taking the difference, if the pulse is homopolar, that is, either a positive pulse or a negative pulse, such as a sensor output pulse, a pulse discrimination level of the pulse is equivalent to a conventional pulse peak value from the ground level. However, if the pulse is bipolar, such as a white noise from a circuit resistance, it is necessary to discriminate voltage between peaks of the pulse from the pulse discrimination level. For this reason, the discrimination level necessary in this case is twice as much as that of conventional discrimination method using pulse peak from ground level.
Therefore, the discrimination level required to count the sensor output limitedly is needed about twice as much as that of the conventional method, and thus the ratio of sensor signal to white noise, that is, the signal-to-noise ratio (S/N ratio), worsens.
A second subject concerns improvement of resistance to noises in the Campbell measurement method. Conventional noise test of a motor, for example, shows that a surge noise with a pulse width of several microseconds is easily induced to the reactor start-up monitoring system.
In the pulse measurement method, this surge noise can be reduced by pulse discrimination according to the above-mentioned digital calculation. On the other hand, in the Campbell measurement method, a measurement band is set as a frequency band from several hundreds of hertz to one megahertz, which is selected according to a form of a sensor output pulse, and in the above-mentioned precedence example, the induction noise is removed by shifting this measurement band. However, since the frequency of the noise that is the easiest to be guided mostly falls in a range of the measurement band, it is difficult to remove the noise completely, and it is necessary to rectify sensor sensitivity because the sensitivity changes slightly.
Generally, a measurement device for measuring dose equivalent is optimized in a radiation incidence window, reaction volume, etc., of a sensor, so that sensitivity characteristics over gamma ray energy of the device may become equal to energy absorption characteristics of a human body. However, it is difficult to make the sensitivity characteristics in agreement correct because the sensitivity characteristics differ according to directions of incidence of gamma rays.
Moreover, as far as accurate conversion of the dose equivalent to a human body is concerned, since energy absorption characteristics differ according to parts of a human body, it is difficult for independent use of the measurement device modified to equalize to the sensor sensitivity over gamma ray energy to evaluate the dose equivalent in each part of a human body. Furthermore, when neutrons other than of a gamma ray, such as a beta ray, are intermingled, a sensor that has rectified its sensitivity by arranging sensor structure cannot estimate such mingled radiations, each of which has absorption characteristics which are greatly different from that of another radiation. Therefore, it must arrange a plurality of measurement systems each of which is used for measuring one radiation exclusively.
Conventionally, in order to solve these subjects, it is proposed and put in practical use to compute energy spectrum of a gamma ray to be converted to the dose equivalent. However, since this technique is based on acquisition of energy information by using pulse height, in a condition in which pileup of pulses is occurred, it becomes difficult to acquire the energy information and thus the accuracy of this technique worsens.
That is, although depending on a pulse width of a sensor output pulse, a maximum of conventional energy measurement is about 1*105 counts per second (CPS). If it is supposed that a minimum of the measurement is one CPS, which must satisfy a response demand, a measurement range goes into about 5 figures. Thus, it is desired to realize a measurement method which enables to measure a dosage in more extensive range continuously.
Japanese Patent Disclosure (koukai) No. H3-183983 shows that dual structure of sensors in a radiation measurement device for measuring dose equivalent in depth of one centimeter improves measurement precision. In this technique, the above-mentioned pileup influence in the pulse measurement method is evaded by means of measuring current. However, sensor structure and processing in this technique are complicated, thus it is desired that they be simplified.